In the formation of glass fibers, molten glass is pulled from a bushing through orifices or bushing tips located on the bottom of the bushing. Originally, the number of bushing tips did not normally exceed 200. However, as the need for glass fibers has increased, bushings have increased in size and now contain 800 to 2,000 bushing tips, or even more. This has led to the use of split bushings which may, for example, contain two sides each side containing up to 1,000 bushing tips, for a total of 2,000 tips. In order to keep the glass molten prior to passing through the bushing tips, it is imperative that the bushing be heated. Temperature control is vital, since the ability to form glass filaments is directly dependent upon the viscosity of the glass which is in turn directly dependent upon the temperature of the glass. With the advent of the split bushing, the problem arose as to how to control the temperature of each side of the bushing individually without the costly necessity of two separate power systems. The present invention provides such a circuit.
U.S. Pat. No. 3,308,270 issued to the inventor of the present invention and assigned to the assignee of the present invention discloses a circuit for controlling the temperature of two separate bushings using one main power transformer. However, some of the problems of the system include the necessity for keeping a bias current within the system, the requirement for a manual setting of the wiper arm, and the need for two secondary transformers within the system.
U.S. Pat. No. 3,803,385 does supply two or more heaters from one main power supply. However, the only temperature control is to turn power on or off completely to a given heater.
U.S. Pat. No. 3,291,969 supplies current to various sections of a single heating element, but uses a pluality of power supplies.
The present invention has eliminated many of the problems associated with the prior art systems and has through the use of solid state circuitry provided a simpler system for controlling two or more heating elements from a single power supply.